[clang-format] Make NamespaceEndCommentFixer add at most one comment

Summary:
Until now, NamespaceEndCommentFixer was adding missing comments for every run,
which results in multiple end comments for:
```
namespace {
  int i;
  int j;
}
#if A
  int a = 1;
#else
  int a = 2;
#endif
```
result before:

```
namespace {
  int i;
  int j;
}// namespace // namespace
#if A
  int a = 1;
#else
  int a = 2;
#endif
```
result after:
```
namespace {
  int i;
  int j;
}// namespace
#if A
  int a = 1;
#else
  int a = 2;
#endif
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D30659

llvm-svn: 297028